1. Field of the Invention
The present invention relates to a call control system for a subscriber transmission device that is located between a transmission path side, which is connected to an exchange, and a subscriber.
2. Related Arts
FIG. 12 is a diagram illustrating a digital signal transmission system. A subscriber transmission device (RT: Remote Terminal) 1 is located between a transmission path side I, which is connected to an exchange, and a subscriber side II. On the transmission path side I, DS1 signals of 1.5 Mbps are transmitted from a digital exchange 3 to an ADM (Add Drop Multiplexer) 2 and are multiplexed to an optical signal of 150 Mbps by the ADM 2, and the optical signal is transmitted across an optical transmission path OC-3 (optical transmission path for 155 Mbps that is specified by SONET standards) for the connection to the subscriber transmission device (RT) 1.
On the subscriber side II, the subscriber transmission device (RT) 1 is connected to a carrier serving area 4 across a narrow band subscriber line 40; a wide band subscriber line 41, for transmission of signals of 45 Mbps, such as DS1 and DS3 signals; or an optical fiber subscriber line 42, each of the specified lines being connected to a local switch or to a specific service.
The subscriber transmission device (RT) 1 comprises an add drop optical multiplexer 10 and a call control system, which provides various subscriber services and which has a function to sort and transmit signals on subscriber lines to exchange devices by using its electronic cross connect function.
FIG. 13 is a block diagram illustrating the conventional structure of the call control system 11 of the subscriber transmission device (RT) 1. In FIG. 13, the add drop optical multiplexer 10 of the subscriber transmission device (RT) 1 is omitted, and only the call control system 11 is shown. Further, representations of the transmission path side I and the subscriber II are simplified, and only their conceptual representations, which are connected to an exchange and to subscribers, respectively, are depicted.
As is shown in FIG. 12, the transmission path side I exchanges signals with the exchange 3, while as for the subscriber II, subscriber lines (channel CH), which are equivalent to 50 subscribers, are connected to a single unit package in the call control system 11. As the number of unit packages of the call control system 11 is increased, the number of subscriber lines (channels CH) that can be connected thereto increases proportionately.
The function of the call control system 11 will now be explained while referring to FIG. 13. The exchange 3 supports a call control message in consonance with a protocol (p). Therefore, the call control system 11 has the following functional blocks.
The function of a call control message terminal section 110 is the transmission of call control messages. A call control processor 111 receives, from the call control message terminal section 110, a notice of the contents that are required by the exchange 3, in consonance with a requested/accepted call by a subscriber, and connects the subscriber to the transmission path.
A call change detector 112 detects changes in a call and the clearance of the call for a subscriber, and requests that the call control message terminal section 110 notify the exchange 3 of the data detected. A transmission path supply main memory 113 is a storage device for the connection to and the clearance of a transmission path for each subscriber. A subscriber service management table 114 is employed to manage the supply condition and the connection form for a transmission path to a subscriber. A new call and clearance condition table 115 receives the latest call and the clearance data for each subscriber by using hardware.
A previous call/clearance condition management table 116 is provided in which old data are stored in order to detect a call change and a clearance, when compared with the latest call/clearance data in the table 115.
The primary operation in the thus structured call control system will now be described.
FIG. 14 is a diagram illustrating the operational sequence for the detection of a call change for a subscriber, and the transmission of the data detected, that is, a call control message, to a corresponding exchange.
In a call change detection process, the call change detector 112 periodically compares all the areas in the new call/clearance (signaling) condition table 115 with those in the previous call/clearance condition management table 116 (step S1).
An example of the new call/clearance (signaling) condition table 115 is shown in FIG. 15. The table 115 has areas in which a call "1" and a clearance "0" are recorded for each subscriber number #1 through #50.
The previous call/clearance condition management table 116 has the same structure as that of the new call/clearance (signaling) condition table 115, and a call "1" and a clearance "0" are recorded for each of the subscribers #1 through #50.
When the new call/clearance (signaling) condition table 115 is periodically compared with the previous call/clearance condition management table 116, changes in the condition can be detected. More specifically, in FIG. 14, when a subscriber makes a call (step S2), the change in the signaling data are reflected in the new call/clearance (signaling) condition table 115. The call condition can be detected by comparing such a table 115 with the previous call/clearance condition management table 116 (step S3).
Then, the transmission of the detected contents to the exchange 3 is requested of the call control message terminal section 110 (step S4). The call control message terminal section 110 edits a call control message in consonance with the data obtained by the call change detector 112, and transmits the edited message to the exchange 3 (step S5).
At this time, the call change detector 112 updates the entries in the previous call/clearance condition management table 116, by copying the detected contents to a corresponding subscriber area (step S6), to prepare for the detection of the next condition change.
FIG. 16 is a diagram showing the operational sequence for the connection of a subscriber with a transmission path upon receipt of a call control message from the exchange, and for the examination to determine whether a contention state exists between subscribers and between transmission paths.
A call control message that indicates a transmission path been supplied from the exchange 3 to a subscriber is edited, and the resultant message is sent to the call control message terminal section 110 (step S11). Upon receipt of the call control message, the call control message terminal section 110 analyzes the message and requests the call control processor 111 connect a subscriber with a transmission path that is designated by the exchange 3 (step S12).
Upon receipt of the connection request, the call control processor 111 searches the subscriber service management table 114 to determine whether or not a contention state exists between subscribers. An example of the subscriber service management table 114 is shown in FIG. 17. For each subscriber, a connection or non-connection (a .oval-hollow. indicates a connected state), a connection form (a corresponding form is indicated by the entry of a .oval-hollow.), and the identification of an ISDN subscriber (a .oval-hollow. is provided in a corresponding column) are recorded in the table 114.
The connection form is categorized as consisting of a dynamic assignment, a semi-permanent assignment or a permanent assignment.
As the outline of a service that corresponds to the dynamic assignment, in consonance with the request/acceptance of a call by a subscriber, the subscriber is connected to a transmission path that is supplied by the exchange 3. Disconnection from the transmission path is also instructed by the exchange 3, in consonance with the clearance of the call by the subscriber. The service for the dynamic assignment is, for example, a general subscriber service, a public telephone network service, or a line switching ISDN service.
To provide service for a semi-permanent assignment, a maintenance operator instructs, via a maintenance system, the connection and disconnection of a fixed, specified subscriber with a designated transmission path for a example, are used for this service.
As for a permanent assignment, a maintenance operation sets an operational form of the maintenance system, and unconditionally determines the correspondence between subscribers and transmission paths to perform connection and disconnection. An operational mode that is specified by protocol, for example, is used for this service.
The call control processor 111 determines whether or not specified subscribers are in contention for a connection (step S13). In this process for determining whether subscribers are in contention, an examination is made to ascertain whether or not a subscriber for which the exchange 3 makes assignments is already connected to a transmission path that is different from the transmission path that is designated by the exchange 3.
The process performed when it is established that subscribers are in contention differs depending on the error process that is specified by the protocol that is supported by the exchange 3.
The determination of whether a contention state is made by referring to the contents of the service data that are held for each subscriber in the subscriber service management table 114, an explanation for which is given in FIG. 17.
The call control processor 111 searches through all the areas for supplying transmission paths in the main memory 113, the structure of which is shown in FIG. 18, and determines whether a state of contention exists in the assignment of transmission paths (step S14).
As is shown in FIG. 18, in the main memory 113 for supplying transmission paths there are areas in which logical address data for transmission path connections are set for each subscriber number. Thus, the occurrence of a contention state involving the transmission paths can be determined by examining whether or not the logical addresses for the transmission path connections that are set in the areas correspond.
More specifically, in the processing for searching for the occurrence of a contention state involving transmission paths, a check is performed to determine whether or not a subscriber other than the subscriber that is specified by the exchange 3 is already connected to a transmission path that is designated by the exchange 3. As well as the process performed when a state wherein subscribers are in contention has occurred, the process performed when a contention state involving the transmission paths has occurred differs in consonance with the error processing specified for the protocol that is supported by the exchange 3.
When the call control processor 111 ascertains that a contention state has not occurred involving subscribers and transmission paths, the subscriber is connected with the transmission path that is designated, from among those that are supplied by the main memory 113, by the exchange 3 (step S15). At this time, the call control processor 111 updates the subscriber service management table 114, in consonance with the contents that are set in the main memory 113 for supplying transmission paths, and matches the contents of the two tables to prepare a succeeding request for the connection of a subscriber with a transmission path.
As is described above, the conventional call control system is realized by the detection of a call change by a subscriber and the transmission of the detected contents (a call control message) to a corresponding exchange, and the connection of the subscriber to a transmission path, in consonance with a call control message received from the exchange, after making an examination to determine whether a contention state involving subscribers and transmission paths has occurred. However, this system is controlled by a call control message for which there is a single, specific protocol, and the system has so small a capacity that only 50 (CH) subscribers can be accommodated.
When many subscribers are to be accommodated, a subscriber transmission device is constituted by combining four of the above described control systems to provide sufficient transmission paths to accommodate 200 (CH) subscribers.
As is described above, the conventional call control system can be located only between an exchange, which supports a call control message in accordance with a single, specific protocol, and a subscriber that is assigned to the exchange. Since a transmission path connection time that is specified by the protocol can not be maintained due to the need to make a determination as to whether or not a contention state involving subscribers and transmission paths has occurred, and due to the load that is imposed because of the need to detect calling, only about 50 (CH) subscribers can be accommodated by one call control system.
It is difficult for such a call control system to increase the switching service types, to enhance the maintenance function upon the request of a subscriber, and to provide a high capacity at a lower manufacturing cost, all of which are subscriber transmission device capabilities that are currently in demand.